1. Field of the Invention
The present invention relates to an ultrasonic flow meter for detecting the flow rate of a liquid based on a propagation velocity difference exhibited when acoustic waves are propagated through the liquid.
2. Description of the Related Art
Heretofore, an ultrasonic flow meter has been known in which, for example, paired transducers are arranged respectively on an upstream side and a downstream side of a conduit through which a liquid flows. Ultrasonic waves transmitted from one of the transducers are reflected by an inner wall surface of the conduit and are received by another of the transducers, and a flow velocity or a flow rate of the liquid is measured based on a difference in propagation velocities of the ultrasonic waves.
In such an ultrasonic flow meter, for example as disclosed in Japanese Patent No. 2793133 (Patent Document 1), a measurement conduit is provided having a supply pipe to which the liquid is supplied and a discharge pipe from which the liquid is discharged. Further, a first measurement head is disposed on one end of the measurement conduit, and a second measurement head is disposed on another end of the measurement conduit. The first and second measurement heads are constituted to function as acoustic wave emitters or acoustic wave receivers. For example, a pulse shaped acoustic wave signal is transmitted from the first measurement head, and the second measurement head receives the acoustic wave signal as an acoustic wave receiver. Next, the first measurement head is switched to operate as a receiver, and by reception thereon of an acoustic wave signal transmitted from the second measurement head, the flow rate of the liquid is measured based on a difference in the propagation velocities of the acoustic wave signals.
However, with the ultrasonic flow meter according to Patent Document 1, a structure is provided in which the liquid supplied to the supply pipe is changed in direction substantially perpendicularly, whereupon the liquid then flows to the measurement conduit. Therefore, by the sudden change in the flow direction of the liquid, a pressure change occurs resulting in turbulence, so that air entrained in the liquid forms gas bubbles in the liquid, whereby such gas bubbles adhere to the inner wall surface of the measurement conduit, which is disposed in confronting relation to the first and second measurement heads. Due to adhesion of such gas bubbles, the gas bubbles interfere with propagation of the acoustic wave signals, leading to deterioration in the accuracy with which the flow rate of the liquid is measured.
Further, with the ultrasonic flow meter disclosed in Japanese Patent No. 3246851 (Patent Document 2), transducers are arranged respectively on opposite ends of an inflow conduit, together with a supply pipe and a discharge pipe, each of which is inclined at a predetermined angle with respect to the axis of the inflow conduit, being connected to both ends thereof. The supply pipe and the discharge pipe are disposed along a straight line and are connected to the inflow conduit through curved pipes, which are bent respectively.
With such a structure, compared to the case of the ultrasonic flow meter according to Patent Document 1, a change in pressure, which occurs when the liquid flows into the inflow conduit from the supply pipe, or when the liquid flows into the discharge pipe from the inflow conduit, is decreased, and the occurrence of gas bubbles is suppressed to a certain extent. However, in this type of ultrasonic flow meter, since the connections of the supply pipe and the discharge pipe with respect to the inflow conduit are not straight, but rather are of a step-like shape, more than a minimal amount of pressure loss occurs, and generation of gas bubbles cannot be entirely avoided.
Furthermore, with the ultrasonic flow meter disclosed in Japanese Laid-Open Patent Publication No. 2002-365106 (Patent Document 3), a pair of ultrasonic transceivers is disposed in facing relation to an outer surface of a tubular body, the ultrasonic transceivers being arranged at an acute angle with respect to the axis of the tubular body. With the ultrasonic flow meter having the aforementioned structure, since the tubular body through which the liquid flows is formed along a straight line, upon flow of the liquid through the tubular body, a change in pressure does not occur, and thus generation of gas bubbles or pooling of liquid is suppressed.